The Global System for Mobile communication (GSM) is a popular communication system having more than 3 Billion subscribers world-wide. Each year there is a significant increase in the number of GSM subscribers, especially in China and India, where subscriber density is very high. Supporting a large number of simultaneous voice calls is problematic for GSM operators. As subscriber growth increases, especially in areas having a high population density, this problem will become more frequent. Accordingly, solutions have been devised to increase the capacity of GSM networks.
One recent initiative to increase the capacity of GSM networks is known as Voice Services over Adaptive Multi-user channels on One Slot (VAMOS). VAMOS can double system capacity and help optimize spectrum efficiency and usage of scarce radio resource, while reducing radio base station power consumption. That is, VAMOS doubles the number of users served by a single radio resource.
In VAMOS, the network assigns the same physical channel (frequency (f) and time-slot (T)) to two different mobile stations (e.g., MS1 and MS2). Therefore, in the downlink, the network assigns the same physical resources (e.g. frequency/time-slot combination) to two different mobile stations, but allocates to each mobile station one of a pair of a training sequences for their respective channel estimation, thereby doubling the system capacity in case of Traffic channels. The training sequence pair is known a priori to both the mobile stations. The network decides to which of the pair of mobile stations higher transmit power should be allocated based on, for example, the near-far position of paired mobile stations and uses the Adaptive Quadrature Phase Shift Keying (AQPSK) modulation technique to introduce power imbalance between the two paired mobiles stations.
Thus, in VAMOS, to achieve the doubling of the system capacity, co-channel interference is introduced intentionally. Before decoding, each mobile station therefore has to use an interference cancellation technique to reject the signal energy corresponding to the signal intended for the other mobile station from the received AQPSK modulated signal. But, for better receiver performance, prior to applying the interference rejection technique, the mobile stations needs to detect the power imbalance introduced by the network.
What is desired, therefore, are methods for use in a receiver for detecting this power imbalance.